Keep block parameter types documented by @!parse stubs

[apiology]

This PR was written by Claude Code on behalf of @apiology.

Based on castwide#1288.

A workspace @!parse stub's block parameter type has no effect when the
gem's own doc already declares a return type:

module Widgetbox
  class << self
    # @return [String]
    def build(&block) = 'x'
  end
end

# @!parse
#   module Widgetbox
#     class << self
#       # @yieldparam config [String]
#       # @return [String]
#       def build(&block); end
#     end
#   end

Widgetbox.build { |config| config.upcase } # Unresolved call to upcase

The method pin itself carries only one of the two signatures, so hover
and completion lose the same information inference does.

combine_signatures_by_type_arity buckets signatures by type_arity
before merging, and a block's declared-parameter count changes that
bucket, so this adds a pass that merges block-informativeness-only
differences first and makes Callable#combine_blocks prefer the more
informative block instead of choosing arbitrarily; two related gaps stay
open (a stub missing @return is discarded wholesale by
combine_signatures, and def self.build paired with class << self
never combines pins at all).